Various spring clutches are known for transferring torque from a drive member to a driven member such as that disclosed in U.S. Pat. No. 4,570,318 to Kish. Spring clutch assemblies are preferred as torque transmitting devices where overrunning conditions would be encountered. Such conditions occur where the driven member attains a higher rotational speed than the drive member and therefore declutching is required to prevent rotation of the driver. Overrunning spring clutch assemblies generally utilize a coil spring which expands radially under a driven load to couple two clutch members, an input member and an output member. Each clutch member includes a bore where the coil spring is located, with approximately one half of the spring in the input member and one half in the output member. The clutch spring is usually fixed at the drive end through press fitting onto an arbor which is attached to the input member. The arbor is coaxial with the input and output members, passing through the hollow core of the spring. The arbor serves as a centering means for alignment and support of the coil spring.
In the non-rotating state, a space is provided between the spring and the inner bore surfaces of the clutch members, except at the ends, where "teaser" coils are in contact with the inner surfaces. These teaser coils ride on the surfaces to actuate the spring. When the input member begins to rotate, in a direction counter to the pitch of the spring coils, the teaser coils dragging against the output member cause the spring to unwind, expanding radially so that it is in driving contact with the bore surfaces along its entire length, transferring torque from the input to output member.
In the overrunning condition, the driven member rotates faster than the drive member, in the direction of the spring pitch, thereby driving the teaser coils to rewind the spring and disengage the driven member from the driver. This may occur, for example, in a helicopter after engine shutdown when momentum maintains main rotor rotation.
While spring clutches have been utilized in many applications, various problems ave limited their usefulness in helicopter aircraft. Attempts have been made to expand the speed range of spring clutches, typically used from 4000 to 8000 RPM, to high RPM applications of up to approximately 30,000 RPM, to increase their compatibility with gas turbine engines. However, severe vibrations have been encountered which may cause spring failure or other damage to the assembly.
The vibrations may occur partially due to the difficulty in balancing the input and output clutch members. Generally, the input and output members are placed in an inboard/outboard relation, with the output member having the input member internally located, providing a series type spring mounting. While both the input and output clutch members are dynamically and statically balanced prior to assembly, each will still have a slight residual imbalance. Once assembled, the input and output clutch members, coupled by the coil spring are dynamically balanced. However, each engagement and disengagement of the clutch spring with the driven member realigns the residual imbalances which could eventually result in the residual imbalances coinciding, providing an intolerable imbalance which generates severe vibration. Should the imbalance occur in the drive mode, with the spring in the expanded condition, the spring coil which traverses the gap between the input and output members may partially enter the gap, misaligning or cocking the spring and causing strain with the potential for spring fracture.
The most serious problem encountered in high rpm operation occurs when the input member travels through a rigid body mode of vibration. A rigid body mode is so called because the assembly components do not bend, but tend to vibrate as a spring/mass system about the supports. Balancing the components alone is insufficient to provide stable operation through a rigid body mode even if the point of maximum deflection is located at a bearing. Normally, rigid body modes can be traversed in operation by assuring that the parts are in good balance and by reliance on the damping inherent in a normal oil lubricated bearing. However, it was found that the maximum deflection in a spring clutch assembly did not occur at the bearing locations, and could not be traversed without exceeding established G force limits, as the mode occurred at high speed and even with lubrication there was insufficient damping.
To stiffen the assembly to reduce deflection, the assembly may utilize increased spread between the bearings. However, this is difficult to accommodate where space is limited, and is also undesirable from a weight standpoint.
Another problem with existing spring clutches is attaining proper spring positioning. Generally, the spring is placed on the arbor using precision measurements, with the arbor then inserted and attached to the input member, usually by pinning. If the coil spring is improperly placed on the arbor, the spring may bind with the input member, inducing undesirable preload, which may fracture the end teaser coils during operation. Similarly, the absence of end spacing for accommodating axial spring displacement on the arbor could cause the spring to bind with the output member during expansion, causing improper clutch operation.
Yet another problem with clutch springs is the tendency to fracture in the transition area between the central coils and the end teaser coils. Generally, the difference in outside diameter of the teaser coils and central coils may cause one or more coils in the transition area to be unsupported when expanded, overstressing those coils and possibly inducing coil failure.
Typically, clutch springs have a central cross-over coil which traverses the gap between the input and output members formed by a near-axial cut during machining. This provides half the axial crossover coil in the input member and half in the output member. However, to achieve an axial cut using a conventional manufacturing method, a small hole is usually left on one side of the finished spring. Under the influence of centrifugal force, the missing material induces an imbalance, producing vibrations which could damage the spring clutch assembly.
When torque is applied to an overrunning clutch spring which does not have an axial cut, the spring may be unsupported as it pushes against the gap between the input and output clutch members. The spring then attempts to unwind into a U-shape and in so doing, may cause the input and output ends of the spring to be displaced radially with respect to each other, essentially cocking the spring. This radial deflection must be minimized to reduce or eliminate the resultant centrifugal imbalance which would be amplified in a high RPM application.